Hydraulic suspension dampers typically comprise a tube filled with working liquid, inside of which a slidable piston is placed. The piston is attached to a piston rod led outside the damper through a piston rod guide, and has a form of a piston assembly comprising rebound and compression valves, which control the flow of working liquid passing through the piston during the rebound and the compression stroke of the damper. Some dampers also comprise a base (bottom) assembly with separate rebound and compression valves controlling the flow of working liquid passing in and out of the additional compensation chamber.
Typically, damping force generated by a damper is related to the speed of the piston and motor vehicle suspensions are designed in order to provide unobstructed displacement of the piston up and down within a tube with respect to some neutral position defined for some nominal load of a motor vehicle in rest.
Unfortunately, this neutral position of a piston varies during daily vehicle exploitation. In case of overloaded vehicle, this neutral position of a damper shall obviously be shifted down the compression chamber of a tube, while in case of an empty vehicle it shall obviously be shifted up the rebound chamber of a tube. In both cases this shift of a neutral position may lead to a situation in which displacement of a piston will exceed its assumed stroke limits leading to a suspension closure or jounce bumper engagement, which in turn affects safety, comfort, durability and noise issues. On the other hand, even for a nominally loaded vehicle, displacement of a piston may also exceed its assumed stroke limits, for example while driving on a rough road or during a road bump.
In order to prevent the contact of a piston at the end of compression stroke with elements at the end of a damper compression chamber, compression stops or bumpers are commonly used. Such stops are mounted to a piston or a base valve and aim to block the flow of working liquid to the compensation chamber thus stopping a stroke of a piston. Such compression stops however do not assure appropriate energy dissipation and their action is unrelated to the speed of a piston in a moment of engaging the stop. Therefore their characteristics do not provide proper comfort to passengers of a vehicle, not to mention passengers' safety.
U.S. Patent Application US 2010/0059321 discloses a position sensitive damper having a second damping piston assembly slidably mounted in the tube, independently from the main piston, and biased to a neutral position. The second damping piston assembly has a damping flow path and a bypass flow path which is normally kept in an open state. When an abnormally strong shock occurs, the displacement of the main piston exceeds its normal displacement span and its obstructing portion is placed into obstruction with the bypass flow path, thereby forcing the working liquid through the damping flow path of the second damping piston assembly.
U.S. Pat. No. 4,768,629 discloses a double-tube vibration damper where the piston acts on an abutment spring when arriving at a predetermined position during compression stroke. The abutment spring acts on a control member. The control member, such as a floating disc, cooperates with a compression passage to reduce the cross-sectional area of the compression passage in response to the piston starting to act onto the abutment spring. In this way a hydraulic abutment function is obtained during compression stroke of the piston. The control member is biased by a return spring towards the abutment spring. The abutment spring acts with its end remote from the control member onto a friction ring, which friction ring is frictionally guided by the inner face of the cylinder. The biasing action of the return spring is larger than the force necessary for moving the friction ring along the inner face of the cylinder.
It has been the object of the present invention to provide a damper that would prevent abrupt stop of a piston at the end of the compression stroke, featuring a simple and economical construction with only a few elements, which would provide versatile tuning options. Yet another object of the present invention has been to provide a damper with a position dependent valve assembly that would not require substantial modification of the remaining elements of a damper and might be employed as an add-on device in dampers already manufactured.